In recent years, significant research efforts have been made worldwide to develop transportation fuels and commodity raw materials from renewable biomass to effectively deal with the depletion of petrochemical fuels and global warming caused by greenhouse gases, and continuous attempts are being made to introduce such applications to the fuel production.
Biomass, which is believed to be a sustainable source of energy, includes lignocellulosic biomass which mainly includes phanerophytes and algal biomass which mainly includes algae that grows in water. Main components of such biomass include: cellulose including glucose, which is a primary nutrient for fermenting strains of the fermentative processes of preparing bioalcohols such as bioethanol and biobutanol; hemicelluloses mainly including pentoses such as xylose, which is not favored by fermenting microorganisms, but is a raw material for xylitol; and lignin which is currently used mainly as a heat source in the biomass production processes, but is getting more attention due to its usability in benzene ring compounds. However, each component of the biomass is delicately fused to one another and interconnected by various chemical bonds, and hence, the biomass in its natural state may not be easily obtained by fractionating the biomass into each component.
Accordingly, in the case of the lignocellulosic biomass, the biomass is first pulverized into a powder to make each component easier for fractionation, and then the powder is subjected to a biomass pretreatment process, which disintegrates the tissue by using various physiochemical methods, followed by enzymatic saccharification using a hydrolase or simultaneous saccharification and cofermentation of resulting products to obtain sugars.
Conventional methods that are generally used for a biomass pretreatment process include autohydrolysis (or hydrothermolysis), dilute acid pretreatment, lime pretreatment, ammonia pretreatment (ARP, etc.), steam explosion, and the like. During the pretreatment process, hemicellulose or lignin from a lignocellulosic biomass is dissolved to expose cellulose, and the lignin is known as the main obstacle that decreases the sugar yield. Previous researches in this field suggest that the dissolved lignin from the pretreatment process may directly inhibit enzymatic activities during an enzymatic saccharification process. Also, the dissolved lignin is readsorbed to a surface of the cellulose during a recrystallization process of the lignin to not only physically block the enzymes from coming into contact with the surface of the cellulose, but also irreversibly adsorb the enzymes to the surface of the lignin and inactivate the enzymes. For these reasons, lignin is known to decrease the conversion rate of cellulose to glucose.
Autohydrolysis (hydrothermolysis) or dilute acid pretreatment in which pretreatment effects are generated due to hydrolysis of hemicelluloses by acid catalysis at high temperature and dissolution and release of a portion of lignin as a water soluble component, has a big difference between pretreatment conditions for maximizing yield of the released hemicellulose and pretreatment conditions for maximizing glucose yield through a final enzymatic saccharification. The two pretreatment steps are typically necessary to maximize the sugar yield. However, if a median value of the two different pretreatment conditions is taken so as to achieve a high sugar yield with only one pretreatment step, the pretreatment may not only produce a large amount of furfural, which is known as an inhibitor of the fermentation strains due to an excessive degradation of hemicellulose, but also decrease the conversion rate of cellulose to glucose.
To resolve the problems, Edgardo et al. have developed a pretreatment method of dissolving lignin by adding an organic solvent during the biomass pretreatment process (see Edgardo et al., Enzyme and Microbial Technology, 2008, 43:214-219; U.S. Patent Publication No. 2010-0159522, Organosolve and ozone treatment of biomass to enhance enzymatic saccharification), however, the method still requires substantial improvements in terms of recovery rate of the solvent and cost of the processes, etc.
Also, as another alternative method of enhancing the sugar yield by enzymatic saccharification after the biomass pretreatment, there is a method of separating a supernatant liquid from a pretreated solid component of pretreated materials, and washing the solid component thus obtained with an excessive amount of warm water to remove various impurities including decomposed materials of lignin (see Charles E. Wyman et al., Bioresource Technology, 2011, Article in press, Comparative data on effects of reading pretreatments and enzyme loadings and formulations on sugar yields from different switchgrass sources). However, this method requires a phase separation process, a number of washing processes, a large scale wastewater treatment process, and the like, and thus, a drastic increase in the production cost is inevitable.
Therefore, the present inventors have endeavored to enhance the sugar yield from biomass and have found that the sugar yield may be increased when a specific additive is used in at least one of the biomass pretreatment process and enzymatic saccharification process, and thus accomplished the present invention.